A half bridge type modular multilevel converter, which is a conventional power converter, includes: a first arm connected between a positive voltage terminal and an AC terminal; and a second arm connected between the AC terminal and a negative voltage terminal, each of the first and second arms including a plurality of unit cells connected in cascade.
Each unit cell includes: a capacitor configured to be charged to a DC voltage; first and second switching elements connected in series between positive and negative electrodes of the capacitor; first and second diodes respectively connected to the first and second switching elements in antiparallel; and a mechanical bypass switch connected to the second diode in parallel. In each of the arms, the mechanical bypass switches of the plurality of unit cells are connected in series.
By controlling the plurality of unit cells of each arm, one of DC power and AC power can be converted to the other of the DC power and the AC power. If a certain unit cell is broken, a mechanical bypass switch of the unit cell is brought into the conductive state to short-circuit the unit cell, thereby continuing the operation of the multilevel converter (for example, see Patent Document 1 (Japanese Patent Laying-Open No. 2011-193615) and Non-Patent Document 1 (“Modern HVDC PLUS application of VSC in Modular Multilevel Converter topology”, K. Friedrich, IEEE 2010, July 2010)).
When a short circuit accident occurs in a DC power transmission line in a DC power transmission system including such a multilevel converter, a large line direct current flows in the first or second switching element of each unit cell (see FIG. 10). By bringing the first and second switching elements of each unit cell into the non-conductive state when a short circuit accident occurs, the first and second switching elements can be protected. However, when the first and second switching elements are brought into the non-conductive state, the line direct current flows in the second diode of each unit cell. Accordingly, a technique of protecting the second diode is required.
Patent Document 2 (Japanese National Patent Publication No. 2009-506736) discloses a technique of protecting the second diode by: connecting a thyristor to the second diode in parallel; turning on the thyristor when a short circuit accident occurs; and diverting a line direct current to the second diode and the thyristor.